This invention relates generally to electronic amplifiers and, in particular, to circuitry for reducing distortion in high-resolution switching amplifiers of the type wherein multiple references are switched to a load in accordance with an input signal.
Multiple-reference switching amplifiers enhance resolution by switching two or more reference voltages to a load. Configurations of this type are described in patent application PCT/US99/26691, entitled xe2x80x98Multi-Reference, High-Accuracy Switching Amplifier,xe2x80x99 the contents of which are incorporated herein by reference.
In order to operate at low distortion, however, a known relationship must exist between the references used. That is, a known voltage or current (as appropriate) ratio must be present between each two references in order to correctly determine suitable duty cycles in conjunction with each reference. Although autonomous, fixed references may be used, considerable distortion can result due to even very small amounts of reference drift or instability.
Thus, there exists a need for a technique whereby correct reference ratios are presented to the load in applicable amplification schemes.
Broadly, this invention is directed to apparatus and methods for generating a secondary reference voltage for a multi-reference amplifier as a function of a primary reference. In so doing, the secondary reference equals a known, fixed portion of the primary reference, thereby minimizing distortion due to variations in voltage (or current) ratio.
In the preferred embodiment, the secondary or low-voltage reference is replaced with an amplifier connected across the load, a digital integrator, pulse-width modulator, filter and buffer amplifier. A zero detector is used to monitor incoming data. When the input data is zero, the integral of primary or high-voltage reference represents pulse-width data of xe2x80x98one.xe2x80x99 This integral is compared with the integral presented to load through the buffer amplifier, such that the high-voltage reference integral commands a non-inverted output. The output of the amplifier across the load drives the digital integrator to produce higher or lower data values at its output, when enabled by the zero detector. The data value output of digital integrator is converted to a pulse-width train by pulse-width modulator, filtered, buffered, and switched to the load as the secondary reference voltage.